[williamdclt]

This is hexadecimal notation for rgb. Each character represents 2 bytes, which encode a value from 0 to 255.

This is a bit more confusing because this is a shorthand notation: you’d actually need 2 hexadecimal characters for 2 bytes (eg #0077ff would be rbg(0, 127, 255)). In this shorthand notation, I think there’s an implied 0 (eg #07f is #0070f0).

So you can’t represent all rgb colour with only 3 characters, but for this use-case it’s fine.

In css you can use either of these 3 notations, for example

[susam]

The 3-digit shorthand for hexadecimal RGB colours dates back to CSS1, if not earlier.

From <https://www.w3.org/TR/CSS1/>:

> The format of an RGB value in hexadecimal notation is a '#' immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This makes sure that white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display.

I'm quite fond of the 3-digit hexadecmial RGB notation. It's a concise way to express the colours I use for web pages or Emacs font locking. In these cases, I rarely need the full 16-million-colour range offered by 6 digits. The 3 digits are usually more than enough, at least to me.

[siriusfeynman]

Every tool I've ever used referred to this as hex (including design tools but maybe I'm sheltered), whereas rgb refers to the 0-255 triples

If someone asked me for a colour in "RGB" they'd be rightly confused if gave them a hex format colour (obviously you can convert between them but that's not what they asked for)

[stavros]

I'm not sure I agree. One is RGB decimal, the other is RGB hex. It's not Lab, or HSV, or any other color space, it's RGB. Whether it's decimal or hex doesn't change that.

[williamdclt]

Hmmm I see what you mean, but it’s more of an abuse of language I think. Both are rgb. If I asked for an rgb id expect any format, it rarely matters (actually id probably expect an hex code _more_, i see it used more often, as its textual representation is well defined whereas the three numbers don’t have a universal textual representation)

[jenadine]

In CSS, #07f is the same as #0077ff. (i.e. double each symbols)

[williamdclt]

Thanks for the correction!